Sputtering, a physical-vapor-deposition technique, is utilized in many industries to deposit thin films of various materials with highly controllable composition and uniformity on any of a variety of substrates. However, for many applications, the size of the desired substrate continues to increase, necessitating the use of larger and larger sputtering targets during the sputtering process. Unfortunately, sputtering targets formed via conventional fabrication methods tends to be too small for many such applications, particularly if the sputtering-target material is a composite (i.e., a substantially uniform mixture of two or more elemental or compound components), as such composite sputtering targets are difficult or impossible to form with a high degree of uniformity by other methods such as rolling. For example, alloys or mixtures of molybdenum and titanium (Mo/Ti) are typically formed into billets for sputtering targets via hot isostatic pressing (HIP) of a mixture of Mo and Ti powders. The largest such billets tend to be smaller than the sputtering-target size desired for, e.g., sputtering of Mo/Ti films on large glass substrates for flat panel displays (FPDs). In order to provide sputtering targets of the requisite dimensions, multiple smaller targets are often positioned in close proximity to each other (but not otherwise joined together) to form a larger target. For example, for use in a “generation 7” sputtering tool, 12 planar plates having dimensions 2700 mm×200 mm×18 mm may be used to form a larger segmented target of approximate dimensions 2700 mm×2400 mm×18 mm.
Such segmented targets present many disadvantages in terms of particle generation (which results in expensive yield loss for the manufacturer) and film nonuniformity. Particle generation may occur preferentially along the edges of the individual sub-targets, and film uniformity tends to decrease as the edges of the target are approached and/or as the edges of the sub-targets are exposed to the sputtering process. Particle generation is a particular problem for FPDs, as each particle generated during the thin-film deposition process can cause a pixel to fail, which in turn has a deleterious impact on image quality and sharpness in the finished FPD.
Similarly, tubular (or “rotary”) sputtering targets are frequently of a segmented design simply because some sputtering materials (e.g., tantalum (Ta) or composites such as Mo/Ti), generally cannot be formed in sufficiently long tubes. For example, in order to make a long rotary target, multiple short cylindrical tiles of the sputtering material are often simply slipped over and bonded to a tubular backing plate made from an easily formable material such as stainless steel or Ti. A single 2.7-meter tube may have six or more tiles, the edges (as many as 12) of which potentially generate contaminating particles. Particle generation is exacerbated in rotary sputtering machines, because such machines typically contain multiple tubular targets. For example, a “generation 8.5” sputtering tool typically contains 12 separate rotary targets, and thus 144 tile edges potentially generating particles.
Techniques such as electron-beam welding have been utilized in attempts to join sub-targets together to form a larger sputtering target, e.g., a composite target of a material such as Mo/Ti. However, electron-beam welding of Mo/Ti sputtering-target sections results in unacceptable porosity in the welded zone due to the relatively high gas (e.g., oxygen) content of the Mo and Ti in the plates. Furthermore, the electron-beam-welded zone tends to have a markedly different microstructure than that of the bulk of the target, which generally results in deleterious nonuniformity in films sputtered from such joined targets.
In view of the foregoing, there is a need for methods of joining smaller sputtering targets to form large joined targets with joints that are mechanically robust and that do not generate particles during sputtering of the joined target.